In connection with pulp pumps, gas following along with the pulp causes problems during pumping therein, especially in centrifugal pumps. The gas in the pulp accumulates in the form of bubbles, which accumulate in front of the impeller and thereby deteriorates the capacity of the pump. This problem can be eliminated by evacuating the gas from the space in front of the impeller of the centrifugal pump to a sealing space behind the impeller and further out of the housing of the pulp pump, for example by means of vacuum pumps of the liquid ring type. These vacuum pumps are often separate pumps, the suction pipe of which is connected to the sealing space in the housing of the pulp pump. Alternatively, they can be designed as a separate vacuum impeller in the form of a liquid ring impeller located on the shaft of the pulp pump, so that common operation for both impellers is obtained. In the last mentioned case, however, the capacity of the vacuum pump is restricted for spatial reasons.
A system with an external vacuum pump comprises a pulp pump, which by a pipe is coupled to a vacuum pump for the evacuation of gas. Such a pulp pump is disclosed, for example, in Swedish Patent No. 467,466. The pipe usually includes a stop valve and a vacuum control means. Normally, flushing water is supplied to the pulp pump. The flushing water has the object of preventing clogging of the pipe to the vacuum pump and of diluting the pulp in the space behind the pulp impeller. The stop valve is controlled by a control valve for the pulp pump, so that the stop valve is opened at a predetermined setting of the control valve of the pulp pump. The vacuum control means is set so that at normal process conditions stable conditions are achieved.
Such a system has the disadvantage that a definite vacuum must be set in spite of the fact that the process conditions can vary. This implies, that an optimum vacuum cannot be maintained in each situation. As a consequence, the system in certain cases can permit pulp fibers to follow along with the gas through the pipe to the vacuum pump, which results both in fiber losses and in possible damage to the vacuum pump. A vacuum pump is constructed to pump gas, and fiber transfer constitutes a weakness, which reduces the reliability of the entire pulp pumping system. In other cases, the pulp pump can operate with much too great an amount of gas in the pump housing, which reduces the efficiency of the pump and causes so much gas to remain in the pulp that disturbances arise in subsequent process steps.